Reflection antenna employing multiple director elements and multiple reflection of energy to effect increased gain



1964 H. w. EHRENSPECK 3,122,745

NNA EMPLOYING MUL REFLEC N ANTE TIPLE D CTOR ELEMENTS MULTIPLEREFLECTION OF ENERGY EFFECT INCREAS GAIN Filed May 1959 United StatesPatent 3,122,745 REFLECTEON ANTENNA EMlLGYING MULTEPLE BKRE CTGRELER'EENTS AND MULTIPLE RE- FLECTION 9F ENERGY Ti EFFECT 1N (IREASEE)GAIN H rrnann W. Ehrenspeclr, 94 Farnham Sh, Belmont, Mass. Filed May11, 1959, Ser. No. 812,565 1 (Claim. ll. 343-819) (Granted under Title35, U.S. Code (1952}, see. 266) The invention described herein may bemanufactured and used by or for the United States Government forgovernmental purposes without payment to me of any royalty thereon.

This invention relates generally to directional antennas and moreparticularly to a modification of slow wave antennas to produce areflection of energy from an array to cause it to traverse the array atleast once before it is radiated and thereby increase gain.

The gain of slow wave antennas depends on the phase velocity of thesurface wave travelling along it and the length of the antenna; however,for a given length there is an optimum phase velocity beyond which thegain decreases, therefore, for adjustment of antennas at optimum phasevelocity, the gain becomes proportional to the antenna length.

The utilization of the concept of this invention whereby the use of areflection arrangement to cause a traverse of at least part of theenergy of an endfire slow wave array back along the array has been foundto increase the effective length of the array and, therefore, cause anin crease in antenna gain. The gain increase thus achieved isaccomplished without extensive modificaiton of the antenna or physicallyincreasing the length.

Accordingly it is an object of this invention to provide a novel methodand means for increasing gain of all types of slow wave antennas.

It is another object of this invention to provide an antenna of one-halfor less of the usual antenna length for a given gain.

it is still another object of this invention to provide at east doublegain for an antenna of a given length over hat previously attainable.

It is a further object of this invention to provide a novel antenna witha very high ratio of front to back radiation.

It is a still further object of this invention to provide a structure toincrease antenna gain while maintaining a feed at only one point.

Another object of this invention involves the provision of a novelendfire array arrangement that allows sharp narrow beam-width patternswith reduced sidelobes.

Still another object of this invention involves the provision of anantenna utilizable at high or low frequencies.

A further object of this invention involves the provision of an antennasuitable for flush mounting.

A still further object of this invention involves the construction of anovel endflre antenna of currently available material that lendthemselves to standard mass production manufacturing techniques and ofless cost than antennas of similar gain.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiments in the accompanying drawings, wherein:

FIGURE 1 is a schematic representation of a conventional endfire antennawith the main lobe of its pattern;

FIGURE 2 is a schematic representation of the backfire antennaembodiment of my invention with the main lobe of its pattern;

BJZZJQS Patented Feb. 25, 1954 ice FIGURE 3 is a schematicrepresentation of the embodiment of FIGURE 2 adapted for flush mounting;and

FIGURE 4 is a schematic representation of a multiple reflection endfireantenna embodiment with the main lobe of its pattern.

Referring to Fl-GURE 1, a conventional endfire Yagi antenna is shownwherein the feed F at one end excites the array and the energy travelsalong the array by means of directors D with a phase velocity slowerthan that of light. Apart from a negligible amount radiated directlyfrom the feeder F, the energy is radiated from a virtual aperture (shownin dashed lines) located at the termination of the array. The concept ofthe virtual aperture is more fully explained in my copending applicationSerial No. 719,698, flied March 6, 1958, titled Endfire Array, whereinthe virtual aperture at the end of an array includes the field at whichpower levels are from maximum to 26 db below maximum. In most endfirearrays a linear reflector R is mounted behind the feeder, as shown inFIGURE 1, to increase the gain in the forward direction. The energytravels along the array in the direction of the arrows and radiates withthe pattern (sidelobes omitted) indicated.

The phase front of a traveling wave lies substantially in a plane at theradiating end of an endfire array when the antenna is adjusted foroptimum gain in the forward direction. The concept of this invention,embodied in the representation labeled FIGURE 2, allows the travelingwave to impinge on and be reflected by a planar reflector M such thatthe wave now travels along the array for a second time in the directiontoward the feed. Then, the array of FIGURE 1 as modified by the use ofthis invention as shown in FlGURE 2 utilizes a feed F, a reflector ll,directors D, and has a virtual aperture V. By placing the planarreflector M at what would normally be the end or" the array, the newvirtual aperture now appears at the feed end of the array. Reflectors Rand M are spaced at their usual positions with respect to the array atapproximately M4. The major part of the energy first travels along thearray as shown by the inner arrows and travels in the opposite directionas indicated by the outer arrows to be radiated with a higher gain,narrower beamwidth pattern, as shown, in a diverse direction to that ofFIGURE 1. The mirror action of the planar reflector M differs from theusual use and action of plane reflectors with linear or slow waveantennas. With a linear antenna a planar reflector causes constructiveinterference between the original antenna wave and the reflected one toincrease the forward gain. Planar reflectors with slow wave structureshave been used in the past to prevent backlobes. However, the wavesimpinging on the reflector in this case are not slow waves since theusual position is behind the feed. The mirror action of reflector M,since there is no direct wave with which to interfere as in the linearantenna case, and since it is situated in the main wave channel of aslow wave structure to reflect the slow wave energy, causes all theenergy in the channel to retravel along the wray thus creating a largeincrease in gain. This increase in gain is explainable by the fact thatthe retravel makes the antenna act like one double its length. In FlGURE2 the gain would be at least equal to a factor of two, except thatreflector R may tend to reduce the gain factor slightly. The narrowpattern with a high ratio of front to back radiation and decreasedsidelobes with the attendant increase in virtual aperture V requires anadjustment of directors D for a new length suitable for an array whichwould normally be double in length. Linear reflector R, it has beenfound, causes a reflection of the energy toward reflector M; however,its effect on the slow Wave returning down the array is to cause onlynegli ible perturbations in the field. Element M is generally flat andshould be as large as the virtual aperture for maximum gain; however, aslightly curved plate corresponding to the plane of the phase front hithe virtual aperture or mesh screen or closely spaced linear elementsmay be used to form this reflector.

FIGURE 3 is an example of the flush mounting of the antenna of FIGURE 2.In this figure, as in all the figures, like elements have the samedesignations. The feed F in this application is enclosed in a hollowtube T which supports the array and its elements and feeds the feederelements which are insulated from tube T.

The backfire antennas of FIGURES 2 and 3 radiate in a direction reversefrom normal with an eflective length of double and a gain increase of atleast 3 db above a conventional endflre array. A modification of thebackfire antenna allows for further gain increases by utilizing multiplereflection.

The multiple reflection concept is embodied in the schematicrepresentation of FIGURE 4 where a feed F feeds an array of directors Din front or which is placed a partial reflector R, While a reflector Mis placed behind the feed, as shown. This structure modifies theprinciple of the backfire antenna in that only a part of the energytravelling along the antenna from the feed end to the output end isreflected by reflector R while the remainder is radiated in the normalendfire direction. As in the concept described relative to the backfireantenna, the reflected portion of the energy travels a second time alongthe antenna but in the opposite direction until it impinges upon itsfeed reflector M. In the multiple reflection antenna, M is a planarreflector which must be of such size as to reflect back as much of thesurface wave energy possible and for maximum gain is of the same size asthe virtual aperture. The energy then travels a third time along theantenna; this time in the normal endflre direction. After reaching theantenna end it is again partly radiated at the virtual aperture V andpartly reflected back, as before. This process continues until all theenergy has been radiated.

Maximum gain of the multiple reflection antenna with a mirror reflectorM of the same size as the virtual aperture requires optimization of twoparameters: the reflectivity of the partial reflector R at the radiatingend, and the phase velocity along the antenna which is accomplished byadjustment of the length of directors D in ac- Qordance with the neweffective length of the antenna.

Thus reversal of elements M and R of the backfire antenna and making Rproduce perturbations in the slow wave produces increased gain over thebackfire antenna. The multiple reflection antenna has greater structuralsimplicity and strength since the feed is located at the same end as thereflector plane.

The partial reflector R or the planar reflector M may be of solid metal,metalized plastic, screening material, or of closely spaced rods eitherparallel or radially mounted. For optimized gain the individual elementsD are decreased from the optimum value for a conventional endfireantenna for phase adjustment.

The multiple reflection principle may be applied to the backfire antennato increase its gain by utilizing a partial reflector rather than alinear reflector in order to cause a partial reflection back along thearray in accordance with the action described relative to FTGURE 4.

Although the invention has been described relative to particularembodiments, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments, forexample, the methods of increasing gain are applicable to all types ofslow Wave antennas such as the dielectric rod, cigar antenna, etc.Furthermore, the teachings of my aforementioned copending applicationare applicable to the backfire and multiple reflection antennas.

I intend to be limited only by the spirit and scope of the appendedclaim.

What is claimed is:

Means for increasing the gain and decreasing the side lobes of amultiple director type of antenna array comprising a planar reflectorpositioned at what would normally be the emitting end of the series ofdirector ele- I ments, for reflecting energy back along said directorelements in reverse sequence, for emission at the end of said arrayWhere the energy traverse began.

References Cited in the file of this patent UNITED STATES PATENTS

